The present invention relates to security documents including a security image composed of individual security image elements and complementary security image elements. When an attempt is made to duplicate a document on which the security image is present, at least some of the security image elements change their appearance on the intended duplicate document, or are altered with respect to some other physical characteristic on the intended duplicate document. The change in appearance, or the altered physical characteristic, of the elements provides an indication that an attempt has been made to duplicate the document.
For example, the security image elements and the complementary security image elements may be designed such that, upon the attempted duplication or reproduction, e.g., by photocopying, either the security image elements or the complementary security image elements are altered while the others remain substantially the same. The elements are arranged such that the attempted duplication results in the formation of a readily apparent warning image on the face of the document. Specifically, when an attempt is made to copy the document, the word "VOID" may appear on the duplicate document. Further, the security image elements and the complementary security image elements are arranged such that the presence of the security image and the security image elements is not readily apparent on the original. Examples of security documents of this type are illustrated in U.S. Pat. Nos. 4,579,370, 5,149,140, 5,197,765, 5,340,159, the disclosures of which are incorporated herein by reference.
The above-described prior art provides a means by which a person charged with the responsibility of reviewing security documents can identify documents that are unauthorized copies of the original security document. Unfortunately, a security document designer utilizing conventional darkroom screening techniques or conventional digital technology to produce security documents including security image elements and complementary security image elements has relatively limited control over the security image. Specifically, there is little the designer can do to influence the regular and repetitive shape, size, placement, angular orientation, and spacing of the image elements utilizing the conventional production methods. The ability to control these factors allows a designer to enhance the performance of security documents.
Accordingly, there is a need for a security document and a method of preparing a security document wherein the image elements of the security image are positioned to minimize the appearance of regular structure, while the basic functionality of the security document is preserved.